The present invention relates to an acrylonitrile polymer capable of providing a fiber having excellent flame resistance, devitrification preventing property and dye-affinity, a process for the preparation thereof, and a modacrylic synthetic fiber made of the acrylonitrile polymer.
Acrylic fibers have various excellent chemical and physical properties such as dyeing affinity, hand touchness and fastness to light. In recent years, a demand for antiflaming of textile goods is rapidly increasing, and in particular, the flammability of acrylic fibers is in significant question. Acrylic fibers are essentially lacking in flame resistance, and accordingly are unsuitable for use in interior goods requiring antiflammability such as curtain and carpet, and clothes for babies, children and the old.
In order to improve this defect, there have hitherto been proposed various processes, e.g. a process in which a flame proofing agent is added to a spinning solution and the spinning is conducted, a process in which a flame proofing agent is incorporated into fibers by post-treatment, and a process in which a mixture of a flame resisting polymer and an acrylic polymer is extruded into a coagulation bath. However, these processes are not satisfactory for the reason that necessity of the use of large amounts of a flame proofing agent decreases the physical properties and hand touchness of acrylic fibers, or the flame resistance decreases due to elution of a flame proofing agent in dyeing and washing, or industrial production by a mix spinning process is difficult due to difficulty in maintaining a mixed spinning solution uniform and stable.
On the other hand, as a process for semipermanently imparting flame resistance to acrylic fibers, there has been carried out a process in which acrylonitrile is copolymerized with a halogen-containing monomer such as vinyl chloride, vinylidene chloride, vinyl bromide or vinylidene bromide. In that case, the larger the content of a halogen in the obtained copolymer, the better the flame resistance. However, the process has a fatal defect that the devitrification preventing property is remarkably lowered with increasing the halogen content, whereby physical properties, processing property and commercial value of the fibers are impaired. The reason is considered to be that since acrylic fibers containing a large amount of halogenated vinyl monomer units are generally prepared by a wet spinning process, fine voids are easy to be formed in fiber structure, and moreover since the softening point of the fibers is low, the fibers are easy to swell in a relatively low temperature range. For such reasons, it is particularly important to prevent the formation of fine voids in fiber structure in a coagulation bath in order to improve the devitrification property.
In case of the so-called acrylic synthetic fibers containing not less than 85% by weight of acrylonitrile, prevention of the devitrification has been generally conducted by copolymerizing with a sulfo group-containing vinyl monomer which also provides the fibers with dyeing sites. However, in case of the so-called modacrylic synthetic fibers which contain a large amount of halogenated vinyl monomer units, it is essentially difficult to form a dense fiber structure and, therefore, not only prevention of the devitrification is very difficult even if a sulfo group-containing vinyl monomer is incorporated, but also the incorporation of a sulfo group-containing vinyl monomer may rather bring about further decrease of devitrification preventing property. It is of urgent necessity for the fiber and textile industry to raise the devitrification preventing property of modacrylic fibers to the degree of the acrylic fibers.
Improvement of the devitrification of modacrylic fibers from the polymerization side has also been attempted. For instance, it is hitherto attempted to use sodium methallyl sulfonate as a sulfo group-containing monomer. However, since the double bond of sodium methallyl sulfonate is active and has a large chain transfer action, the polymerization reaction is disturbed or the degree of polymerization of the produced polymer becomes small, and accordingly it is difficult to prepare industrially useful copolymers. Even if these defects are mitigated by decreasing the amount of use, satisfactory synthetic fibers are not obtained, because the devitrification is not sufficiently improved and moreover the dyeing affinity is low.
It is also attempted to improve the devitrification of a fiber made of an acrylonitrile polymer containing halogenated vinyl monomer units by employing a high reactive sulfo group-containing vinyl monomer such as sodium styrene sulfonate. Since the reactivity of such a sulfo group-containing monomer is high, a copolymer having a high hydrophilic property is produced in large quantities in an early stage of the polymerization or in a short period of time after the addition of the monomer, when a monomer mixture containing the sulfo group-containing monomer with other monomers such as acrylonitrile and a halogenated vinyl monomer is subjected to the polymerization or the sulfo group-containing monomer is added to the system at one time in the course of the polymerization. In case of preparing a synthetic fiber from the thus produced polymer, the devitrification can be improved in its own way. However, the obtained fiber has a decreased dyeing affinity. The reason is considered to be that the portion of the polymer having a high hydrophilic property is eluted during washing with water in a post-treatment step of the polymer or at the time of coagulation or washing with water in a spinning step.
It is an object of the present invention to provide an acrylonitrile polymer capable of providing a fiber having excellent flame resistance, devitrification preventing property and dyeing affinity.
A further object of the invention is to provide a process for preparing the acrylonitrile polymer.
Another object of the invention is to provide a modacrylic fiber having excellent flame resistance, devitrification preventing property and dyeing affinity.
These and other objects of the present invention will become apparent from the description hereinafter.